What is the point of hiding a webpage until load?

I have noticed that whenever I go to namecheap.com, the page loads abnormally. Instead of the gradual loading like most websites, namecheap stays completely blank for a few seconds, and then suddenly everything appears. I got curious, and I looked at the page source and found this script:

(function(a,s,y,n,c,h,i,d,e){s.className+=' '+y;h.start=1*new Date;
                                h.end=i=function(){s.className=s.className.replace(RegExp(' ?'+y),'')};
                                (a[n]=a[n]||[]).hide=h;setTimeout(function(){i();h.end=null},c);h.timeout=c;
                                })(window,document.documentElement,'async-hide','dataLayer',4000,
                                {'GTM-544JFM':true});

Which goes through all the elements in the page, and removes the async-hide CSS class from them. There is also a CSS rule: .async-hide { opacity: 0 !important} that hides elements with this class.

To me, this seems very bad from a UX stance, but I am wondering if there is any good reason to do this?

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